Oscillator synchronizing circuit with variable pull in range



Nov. 29, 1960 A. PoLLAK 2,962,666

oscILLA'roR sYNcHRoNIzING CIRCUIT WITH VARIABLE PULL-IN RANGE Filed ont.e, 1959 INVENTOR Alfred Po'llak ATTORN EY nted States `QSCILLATORSYNCHRONIZING 'CIRCUIT WITH 'VARIABLE 'PULLJNRANGE .Alfred .Pollak,Hannover, Germany, assigner to VTelle- `funken G.m.b.H.,;Berlin,fGermany Filed'OcL ,6, 119591Ser.' No. ,$44,682

VClaimspriority, application-Germany iOct. 9, 31958 .ZLClaims.(,Cl..331-1'1) .Thepresent invention lrelates -to improvements `in cir-.cuitsfoipulling-in the oscillationtfrequencyofanoscillatoratosynchronize` it with externally tsupplied synchronizingpulses.

A system has been suggested in copending U.S. application,Serial,No.'763,961,liledSeptember 29, 1958, of the same assignee asthepresent application, for synchronizing an oscillator `with asynchronizing `pulse sequence :by using a phase comparisonsystemtiniwhichxthe'pull-in range-is varied as a function of'ta-.controlvoltage de- :rivedy'fromthe coincidence of synchronizing tpulses ,and

oscillationtcomparisonupulses. l.Such 'a circuit has the advantage thatthe pull-in range can be designed very small for the normal operation atsynchronism, whereby an extremely small susceptibility to disturbancesin the system is obtained but, wherein, as soon as this circuit fallsout of step, the pull-in range is automatically expanded by means of acontrol voltage derived from the coincidence circuit.

It is an object of the present invention to provide such synchronizingsystem with minimum complexity, but with great eiciency.

It is another object of the invention to provide, in connection with adamping resistance between the phase comparison circuit and theoscillator, a switch controlled by the degree of phase coincidence toexpand the pull-in range whenever the oscillator falls out ofsynchronization in such a manner, that the whole control circuit iscaused to oscillate until synchronization is restored.

In a preferred embodiment of the invention, the switch comprises a diodehaving a small resistance at synchro* nism, said diode being blockedwhen the synchronism is lost, thereby reducing the value of a resistancewhich is shunted across the diode, said resistance determining theextent of expansion of the pull-in range, i.e. the amplitude of theoscillation of the whole control circuit.

Still further objects and the entire scope of applicability of thepresent invention will become apparent from the detailed descriptiongiven hereinafter; it should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

In the drawings:

Figure l is a schematic diagram showing a circuit according to thepresent invention;

Figure 2 is a modified schematic diagram;

Figure 3 is a schematic diagram of a working embodiment of theinvention.

The embodiment of Figure l shows a control circuit comprising a phasecomparison circuit 1 a filter 2, an oscillator 3 and a comparisonfeedback path 4, said control circuit serving to indirectly synchronizethe oscillator 3 with an externally supplied synchronizing pulseysequence 5. Such control circuit produces a D.C. con- 2,9516 AliterrtedNov. 29, Y19,60

trol voltage in a manner known perse, whichistaiune tion of the phasevcondition ofthe synchronizing tpulse ysequence 5 and a comparisonoscillation 6 lderived from the oscillator, said D.C. control voltagebeing Ausedffor correcting thefrequency of theoscillator 3after beingfiltered by meanstof the filter 2. In this case, thesteep- Vness o-fthe-controllcharacteristic ofthe circuit described so far istdetermined,among` other factors, `by the :design t of the ilter2 `and the-amplitudei of the comparison oscillation. In order` to.avoid self-,oscillation insuch a feedback-circuit, the filter 2 is provided with` a time Vconstantlcircuit, including the members 7, 8 and 18,.forsmoothing .pull-in`range and susceptibility to disturbances, that brief spuriousdisturbances `cannotfcause the oscillator to fall out 1 of step.

fln -order totbe able f to -restore ysynchronism :after Ythe oscillatorhas fallenvout of step` through allarge frequency deviation o'f,1forexample, i500 cycles, a :diode 9 is shunted across `,the lresistance 8ofthe attenuation `circuit 7, S and 18, said resistance affecting thetime` constant of the control circuitwhereinsaid diode is blocked oropened by a second coincidence circuit, including a tube or triode 10,as a function of the coincidence of the synchronous pulses 5 and acomparison pulse sequence 11 derived from the oscillator 3. The triode10 is used as a second phase comparison means to generate a secondcontrol voltage used as the switching voltage, said triode principallyoperating like a trigger control. The synchronizing pulses are fed tothe control grid of this tube via an RC circuit 12 in such a manner,that a peak rectitication occurs at the peak value of the synchronizingpulses. Also positive pulses 14, representing the oscillator frequency,are fed to the anode of the triode 10 via a condenser 13. Such pulsesare available, for example, in a television receiver at the linedeflection transformer during the return sweep. If the triode grid isbiased conductively, the anode and cathode path represents a diode withrelatively high resistance. The return sweep pulses 14 are rectied inthis diode and an average D.C. voltage appears across the anoderesistance of the triode, the magnitude of this D.C. voltage dependingupon the anode resistance, upon the positive biasing voltage on the gridand upon the amplitude of the return sweep pulses. The `rectifiedvoltage is filtered by the iilter 15, 16, and is fed to the cathode ofthe diode 9 via a resistance 17. As long as phase coincidence existsbetween the synchronizing pulses 5 and the comparison pulses 14, anegative voltage is generated at the anode of the triode 10, saidvoltage biasing the diode forwardly. In this case, the negative voltageis selected to approximately equal such value, that the internalresistance of the diode amounts to about 800 ohms. When the phaserelation between the synchronizing pulses S and the comparison pulses 14is out of step, the resistance of the diode path through the anode andcathode of the triode becomes Very high and the rectification of thepeaks of the pulses ceases almost entirely. Therefore, a positivevoltage appears which is obtained from the voltage division across theresistances 15, 8, 17 and 9. As a result of this, the cathode of thediode 9 is biased positively and cut off. Thus, the attenuation circuit7, 8 and 18 has a resistance resulting from the shunting of theresistances 8 and 17. In order to avoid hunting during pull-in, it issuitable to select the time constant of the circuit components 15, 16and 17 serving to lter the switch voltage relatively large.

Itis possible to generate the switch voltage for the diode 9 by means ofa diode 20 rather than a triode 10. Such circuit is shown in Figure 2and is suitable, due to its reduced complexity and expense, particularlyif amplication of the switch voltage is not necessary. In the circuit ofFigure 2, the cathode of diode 20 is grounded by way of a resistor 21,while the anode is connected to the operating voltage VB by way of aresistor 22. When the positive synchronizing pulses 5 fed to the anodeof the diode 20 are in phase with the negative comparison pulses 14, theinternal resistance of the diode is small, so that an intermediatenegative D.C. voltage appears across the anode resistance, therebyrendering the diode 9 conductive. As soon as the-impulses are out ofphase with each other, the diode 20 is rendered non-conductive, therebyproducing a positive switching voltage which depends on lthe particularvalues of the resistors 22, 15, 17 and 8,

these resistors acting as a voltage divider. The switching voltage isthus obtained in a manner similar to that produced by the circuit ofFigure 1.

Figure 3 illustrates a working example of a circuit resembling thatschematically shown in Figure 1, showing circuit values. The circuit ofFigure 3 differs from that of Figure l only in that the capacitor 16 isconnected to the positive operating voltage VB instead of to ground, andthat an additional very small capacitance is connected in parallel withthe diode 9, the operation of the circuit of Figure 3 being identical tothat of Figure 1.

I claim:

1. A system for synchronizing an oscillator with a synchronizing pulsesequence comprising, means for deriving a comparison sequence from saidoscillator; st phase comparison means receiving said sequences anddelivering a control voltage proportional to the degree of coincidence;oscillator control means receiving said control voltage and restoringcoincidence by pulling the oscillator frequency into step with thesynchronizing pulses; filter means interposed between said rstcomparison means and said oscillator control means and comprising aresistance for damping the control circuit; second phase comparisonmeans receiving said sequences and delivering a second control voltageproportional to the degree of coincidence; switching means connectedwith said resistance for altering its value thereby causing the controlcircuit to oscillate freely, said switching means being connected toreceive said second control voltage.

2. In a system as set forth in claim 1, said switching means comprisinga diode shunted across at least a part of said resistance, and saidsecond control voltage being connected to said diode so that theconductivity of the latter is switched according to the degree ofcoincidence of said sequences.

References Cited in the tile of this patent UNITED STATES PATENTS2,730,616 Bastow Ian. 10, 1956 2,764,681 Howell Sept. 25, 1956 2,828,419Gruen" Mar. 25, 1958 2,932,793 Smith et al. Apr. 12, 1960 i i l l

